156 ARCHICELOMATA. 
CHAPTER XV. 
ARCHICG@LOMATA. 
ASTERIAS, BALANOGLOSSUS. LOPHOPUS. SAGITTA. WALDHEIMIA. 
I.—ASTERIAS. 
PHYLUM ARCHICELOMATA. 
Sus-PHYLUM ECHINODERMATA. 
CLass ASTEROIDEA. 
Fig. 89.—ASTERIAS RUBENS x }. 
On the left the oral surface is seen with the five ambulacral grooves and 
tube-feet ; on the right is the aboral surface with the madreporite 
between the two lower arms. 
Asterias rubens (the starfish) is one of the commonest 
marine littoral animals. The body is of a dull yellow-red 
colour, flattened and produced into five equal-sized arms. 
This gives an external appearance of axial 
symmetry, but it will be seen that it is really 
plano-symmetric. For purposes of description, 
the five axes of the arms are termed radii and the five 
radii between them are called the znter-radit. In the 
External 
Features. 
